1. Field of the Invention
The present invention relates to a magnetic head and, more particularly, to a magnetic head which utilizes a magnetic optical effect to reproduce magnetic discs.
2. Description of the Prior Art
Conventionally used disc type recording media include magnetic discs (floppy discs and hard discs), optical discs (compact discs and laser discs) and magnetic optical discs. Of these discs, the magnetic disc has data recorded, reproduced and deleted thereto and therefrom through the use of a magnetic head. The recording reproducing and deleting of the data to and from the magnetic disc are most often performed using what is known as the magnetic induction method. One characteristic of the magnetic disc is its ability to have data overwritten on it. Because it relies on the magnetic induction method, the magnetic disc is required to have a track width as wide as tens of .mu.m. It is difficult to reduce the track width as long as the magnetic induction method is utilized. Today, studies are under way on the so-called MR (magnetic resistance) head that operates on a magnetic resistance effect. However, even if the MR head is practically implemented, the track width involved is still expected to measure as large as several .mu.m.
On the other hand, the optical disc and magnetic optical disc each measure 1.6 .mu.m in track width. The small track width intrinsically affords the optical or magnetic optical disc a high-density, high-capacity data storing capability. On the optical or magnetic optical disc, the diameter of a recording pit may be as small as 0.6 .mu.m when the laser beam used has a wavelength of 780 nm. If the wavelength of the laser beam is shortened, the spot diameter thereof may be made correspondingly smaller. This in turn will boost the recording density on the disc. Given the present state of the art, however, the shortwave laser beam ranging from the blue to outside the near velvet region is difficult to obtain; it is not yet practical to improve the recording density of the disc through the shortening of the laser beam wavelength. Unlike compact discs or laser discs, the magnetic optical disc has one distinct advantage: the latter disc allows data to be overwritten repeatedly on it. However, the speed at which to overwrite data or to gain access to the magnetic optical disc is yet to be improved for more practical use.
One notable property of the magnetic head is its ability to reduce the recording wavelength easily to 0.6 .mu.m or even less by shortening its gap length. If the biggest disadvantage of the magnetic disc, i.e., the large track width, is somehow resolved illustratively by an arrangement implied above, the recording density of the magnetic disc will be significantly improved.
With the magnetic head, the thickness of its magnetic layer facing a magnetic disc represents the track width. In this case, with the eddy current loss taken into account, a magnetic layer several .mu.m in thickness is expected to provide better I/O characteristics in the high frequency range (&gt;10 MHz) than a magnetic layer tens of .mu.m thick. However, as described, the magnetic induction method based on a coil arrangement makes it very difficult to reproduce data from tracks whose width is several .mu.m or less. There is a possibility, as described, that the use of the MR head will allow data to be reproduced from tracks several .mu.m wide. But this is the limit; it is difficult to reproduce data from tracks of smaller widths using the prior art.
Of the methods for detecting the magnetized state of a magnetic substance, the method that works with the smallest magnetic substance thickness is that which utilizes magnetic optical effects such as the Kerr effect. This magnetic optical effect is a phenomenon in which a linearly polarized light beam incident on a magnetic substance has its plane of polarization rotated positively or negatively depending on the magnetized state of the substance. Although there have been reports on applying the magnetic optical effect to detecting the magnetized state of the magnetic head, all such applications are based on the longitudinal or traverse Kerr effect. Because the Kerr rotation angle derived from the longitudinal or traverse Kerr effect is intrinsically small, the reproduced signal output available thereby is low. This means that the use of the Kerr effect is not suitable in implementing a practical method of reproducing data from the magnetic disc.